WHEN Rachel Crowther was born almost 16 weeks early, her parents were told she had only a 5 per cent chance of survival and if she did live, she most likely would have brain damage.

But Rachel, who weighed just 669g as a newborn with a head no bigger than a tennis ball, has defied medical expectations, returning to the Royal Brisbane and Women's Hospital yesterday to celebrate her 10th birthday.

Her parents describe her as a highly intelligent little girl who wins academic awards at the Blenheim State School, near Laidley in the Lockyer Valley, west of Brisbane, where her favourite subject is maths.

"When she was born, the doctors told us to take lots of photos because there's a good chance that's all you'll take home," her father Colin said.

Out of seven babies being cared for in the same room at the RBWH's Grantley Stable Neonatal Unit, including Rachel, Mr Crowther said only three made it home.

"It becomes very real when the baby beside yours passes away," he said.

"Luck was on our side and we had good people helping us.

Rachel Crowther poses at the Neonatal Ward at the RBWH on Thursday, October 12, 2017. Rachel celebrated her 10th birthday on the ward. Picture: Claudia Baxter/AAP

"The staff here did an amazing job. Without these people, we would not have taken her home."

Rachel's Mum Rhonda recalls a couple of "touch and go" nights in the RBWH when she feared her firstborn would not survive until the morning.

"They were dark nights for us but she made it through," Mrs Crowther said.

"She now takes on every life challenge just like when she was born. She's going fine."RBWH director of neonatology Pieter Koorts said babies born at 24 weeks' gestation a decade ago were "right on the limit" of viability.

These days, about 60 per cent of "24-weekers" and 20 per cent of babies born at 23 weeks survive.

"For babies born at 23, 24 weeks it's still a grey area where we let parents, after proper counselling, make the decision about whether they should be resuscitated," Dr Koorts said.

"I don't know what the limit of gestation will be eventually, but currently for us it's 23 weeks under Queensland guidelines."

He said many babies born at such extreme levels of prematurity faced major disabilities.

Nurse Colette McIntyre was one of the RBWH staff members who cared for Rachel and her parents a decade ago.

"I haven't had the pleasure of meeting one of our babies 10 years later," she said after watching Rachel cut her 10th birthday cake.

"It's beautiful to see. It's a privilege looking after these little kids."